Electrophotographic toner particles typically comprise a mixture of a polymer and very fine pigment particles. The toner particles may be made by dispersing the pigments in an organic solvent, such as ethyl acetate or methylene chloride, along with the dissolved polymer and other addenda followed by droplet formation in an aqueous phase and removal of the solvent to solidify the particles (see, e.g., U.S. Pat. No. 4,833,060). Another technique for making electrophotographic toner particles is to disperse pigments in ethylenically unsaturated monomers along with other addenda followed by droplet formation in an aqueous phase and suspension polymerization of the ethylenically unsaturated monomers to solidify the particles (see, e.g., U.S. Pat. No. 4,965,131).
Reducing the pigment's particle size prior to incorporation into a toner particle is desirable in order to increase the covering power of the pigment and therefore reduce the amount required. In view of the electrophotographic toner particle manufacturing procedures described above, it is desirable to mill the pigment particles directly in the organic solvent used to dissolve the toner particle polymer, or in the monomer solution used to form the toner particle, thereby eliminating the need to separate the milled pigment from the liquid milling medium continuous phase prior to formation of the toner particle. The use of conventional organic solvent insoluble milling media such as steel, ceramic or glass beads in an attrition mill to reduce the pigment particle size, however, suffers from several problems. First, excessive levels of metallic, ceramic or other contamination usually result. Metallic contamination is particularly undesirable in an electrophotographic toner where careful control of the toner particle's charging properties is required. Second, while the use of very fine milling media can result in desirably smaller pigment particles, it is generally difficult to obtain metallic, ceramic or glass milling media of a size smaller than 100 mm.
The use of polymeric milling media to grind materials down to a very small size is known, such as described in U.S. Pat. No. 5,478,705, which discloses milling of compounds useful in imaging elements using polymeric milling media, preferably based upon crosslinked or non-crosslinked polymers made from acrylic and styrenic monomers. U.S. Pat. No. 5,500,331 teaches the advantages associated with the use of milling media smaller than 100 .mu.m when milling compounds useful in imaging, and specifically discloses the use of polymeric milling media of such size. While both dry and wet milling is possible when using polymeric milling media, wet milling is generally preferred.
It would be desirable to be able to effectively mill electrophotographic pigment particles using polymeric milling media directly in the organic solvent used to dissolve the toner particle polymer, or in the monomer solution used to form the toner particle, thereby eliminating the problems associated with the use of steel, ceramic or glass beads as the milling media. Such organic liquid media, however, are generally effective solvents for useful polymeric milling media polymers, such as polymers made from acrylic and styrenic monomers, while liquid dispersion medium which does not dissolve polymeric milling media is generally recommended when using polymeric milling media. U.S. Pat. No. 5,478,705, for instance, states that the preferred liquid dispersion medium is water (col 5, line 15), and while additional useful liquid dispersion mediums are indicated (col 4, line 19), all are not significant solvents even for uncrosslinked polystyrene. The use of polymeric milling media in liquid dispersion media comprising good solvents for the polymers which make up the milling media would not be expected to be effective, even if the polymers were sufficiently crosslinked to make the polymers insoluble in the liquid media, due to the plasticizing, or softening effect of the solvent on the polymer.
Additionally, it would also be desirable to provide dispersions of other solid compounds useful in imaging elements in a liquid continuous phase which comprises a good solvent for polymers which would make effective milling media when preparing solvent based coating compositions containing such compounds. The solvent based coating compositions must have unique combinations of properties such as volatility, viscosity, surface tension, etc., to enable effective coating processes, which properties may be advantageously met by liquids which are frequently good solvents for many polymers.
Accordingly, it is an object of the invention to be able to provide a process for forming a dispersion of small solid particles in a liquid dispersion medium continuous phase which comprises a good solvent for polymers which would make effective milling media.